1. Field of the Invention
The present invention relates to power supply circuits for magnetic bearing devices. The invention relates more particularly to a power supply circuit for a magnetic bearing system capable of maintaining the function of the magnetic bearing with regenerative electric power generated by the rotation of a motor without using a backup battery during the power failure, in a magnetic bearing spindle for use in a turbo molecular pump for example.
2. Description of the Related Art
FIG. 1 includes a cross sectional view of a conventional magnetic bearing spindle together with a block diagram for a control portion thereof. In FIG. 1, provided in a housing 21 are an upper radial magnetic bearing 22, a lower radial magnetic bearing 23, and a thrust magnetic bearing 24 for supporting a spindle 31. A motor 25 is provided in housing 21 between upper radial magnetic bearing 22 and lower radial magnetic bearing 23 in order to rotate spindle 31. Radial position sensors 26 and 27 for detecting displacement of spindle 31 in the radial direction are provided in the vicinity of upper radial magnetic bearing 22 and lower radial magnetic bearing 23. A thrust position sensor 28 for detecting displacement of spindle 31 in the direction of thrust is provided in the lower part. In addition, bearings 29 and 30 are provided in housing 21 for supporting spindle 31 in case that spindle 31 is not magnetically supported by magnetic bearings due to any abnormality. Detection outputs from radial position sensors 26, 27 and thrust position sensor 28 are provided to a magnetic bearing control circuit 42, which controls upper radial magnetic bearing 22, lower radial magnetic bearing 23 and thrust magnetic bearing 24.
Magnetic bearing control circuit 42 is provided with DC voltage produced by rectifying and smoothing commercially available AC voltage, the DC voltage drives upper radial magnetic bearing 22, lower radial magnetic bearing 23 and thrust magnetic bearing 24, and spindle 31 is magnetically supported by the magnetic force of an electromagnet and rotates by the driving force of motor 25.
Once the commercially available power supply fails, however, the magnetic force of the electromagnet will be lost. Therefore, backup batteries have been used in order to maintain the magnetic force of the electromagnet during the power failure. Use of such backup batteries, however, necessitates the maintenance of regularly exchanging the batteries due to the deterioration (the end of useful life) of such batteries. For this reason, a power supply circuit capable of maintaining the magnetic force of the electromagnet with regenerative electric power generated by the inertial rotation of motor 25 during the power failure is recently used.
FIG. 2 is a block diagram showing a power supply circuit for conventional magnetic bearing systems. In FIG. 2, AC input voltage provided to a transformer 1 is lowered, and the lowered AC voltage is rectified at rectifying circuit 2 and smoothed at a smoothing circuit 3 to be DC voltage. The DC voltage is supplied to an inverter 4 included in motor driving circuit 41 shown in FIG. 1, and inverter 4 provides motor 25 with power.
The AC input voltage is also converted into DC voltage by an AC/DC regulator 7 for input to a switch circuit 8. The DC voltage smoothed at smoothing circuit 3 is also provided to a DC/DC converter 10. DC/DC converter 10 converts regenerative electric power generated by the rotation of motor 25 during the power failure into prescribed DC voltage. The DC voltage output from DC/DC converter 10 is provided to switch circuit 8. Switch circuit 8 selects the DC voltage from AC/DC regulator 7 during the normal operation and selects the DC voltage from DC/DC converter 10 at a detection of power failure by a power failure detection circuit 11. The selected DC voltage is supplied to the magnetic bearing through magnetic bearing control circuit 42 shown in FIG. 1 as power supply and is converted into DC voltage to be applied to magnetic bearing control circuit 42 by a DC/DC converter 9.
In the power supply circuit for magnetic bearing system shown in FIG. 2, during the normal operation, AC input voltage is lowered at transformer 1, rectified at rectifying circuit 2, and smoothed into DC voltage by smoothing circuit 3 for supply to inverter 4, which drives motor 25 to rotate. AC input voltage is converted into DC voltage by AC/DC regulator 7, supplied to the magnetic bearings through switch circuit 8 as power supply, and converted into DC voltage for magnetic bearing control circuit 42 by DC/DC converter 9.
When AC input voltage fails, power failure detection circuit 11 instantly detects the failure and switches switch circuit 8 to the output side of DC/DC converter 10. Inverter 4 stops operating with the DC voltage produced by rectifying the AC voltage no longer supplied, but motor 25 makes inertial rotation and functions as a generator, and regenerative electric power generated is provided to DC/DC converter 10 through inverter 4. DC voltage output from DC/DC converter 10 is supplied to the magnetic bearings as power supply through switch circuit 8, and DC voltage is provided to the magnetic bearing control circuit by DC/DC converter 9.
Since in the power supply circuit shown in FIG. 2, AC/DC regulator 7 can maintain DC voltage at a prescribed level during the failure of AC input voltage only for several 10 msec, within that period power failure detection circuit 11 must detect the failure and switch circuit 8 must be switched such that the input of DC/DC converter 9 is connected to the output side of DC/DC converter 10. The provision of power failure detection circuit 11 and switch circuit 8 makes the structure complexed.